Molecular Dynamics Studies on the Effect of Surface Roughness and Surface Tension on the Thermodynamics and Dynamics of Hydronium Ion Transfer Across the Liquid/Liquid Interface

Abstract

Molecular dynamics simulations are used to examine the effect of surface roughness and surface tension on the transfer of the classical hydronium ion (H3O+) across the water/1,2-dichloroethane interface. Free energy of transfer, hydration structure, and dynamics as a function of the ion location along the interface normal are calculated with six different values of a control parameter whose variation modifies the surface tension without impacting the bulk properties of the two solvents. Transfer of the classical hydronium ion across the water/1,2-dichloroethan interface involves the cotransfer of three hydration shell water molecules, independent of the surface tension. However, as the interaction between the two liquids weakens, a rise in interfacial tension and decrease in intrinsic water fingering and capillary fluctuations result in fewer water molecules cotransported with the ion in the second shell and a reduction in the length of the finger that the ion is attached to, consistent with the reduced size of the second hydration shell. First shell water residence time and lateral ion diffusion constants vary with the surface tension in a way that is consistent with the abovementioned structural insight.

On the mechanisms of ion adsorption to aqueous interfaces: air-water vs. oil-water

“…The ion can be stabilized at the interface by a nearly complete hydration shell. The water molecules in the hydration shell are free to rapidly exchange with other nearby water molecules ( 57 ). Only when the ion is further pushed into the oil phase does this process become less likely and the free energy rises rapidly.…”

Characterizing Anion Adsorption to Aqueous Interfaces: Toluene–Water versus Air–Water

“…Both theory − and surface selective spectroscopies have been used to study liquid–liquid interfaces. For example, vibrational sum-frequency generation (VSFG) has revealed details on surface structure by monitoring changes in the OH-stretching region of water. − The Richmond group has published an extensive study using VSFG, of the CCl 4 –water interface in the presence of various surfactants, biomolecules, and ions.…”

Reparameterization of Polarizable Force Fields for Studying Ion Transfer across Liquid–Liquid Interfaces

“…When the ion is transferred to the organic phase, the ion can be accompanied by its hydration shell water molecules as shown in Figure c. − Such molecular details are important for the optimization of the phase-transfer catalyst as previous experimental and theoretical studies suggested that the hydration state of the ion can strongly affect the reactivity. ,, …”

“…25,30,31 Free-energy change of the ion transfer ΔF, which is the minimum amount of work required to transfer the ion from one phase to another, is one of the key thermodynamic quantities obtained in both experimental and computational studies. [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]32,33 By MD simulations, Dang 21 studied the freeenergy change of Cl − transfer across the water−dichloromethane interface using polarizable models. They found that the free-energy change calculated from their MD simulations, 14(±2) kcal/mol, is in reasonable quantitative agreement with the experimental value, 10(±1) kcal/mol.…”

“…In addition to the experimental efforts, molecular simulations have significantly contributed to our understanding of ion transfer across liquid–liquid interfaces on a molecular scale. Following the pioneering work by Benjamin, , molecular simulations have unveiled the underlying ion-transfer mechanism. − In particular, it has been reported that the ion transfer across the water–oil interface can cause significant interface fluctuations by forming a so-called “water finger” − in molecular dynamics (MD) simulations. However, because of the relatively small surface fluctuations when compared to the experimental precision, water fingers have not been observed experimentally.…”